Conventional drilling typically uses single wall jointed drill pipe or single wall coiled tubing with a drill bit attached at one end. Weighted drilling mud or fluid is pumped through a rotating drill pipe to drive the drill bit to drill a borehole. The drill cuttings and exhausted drilling mud and fluid are returned to the surface up the annulus between the drill string and the formation by using mud, fluids, gases or various combinations of each to create enough pressure to transport the cuttings out of the wellbore. Compressed air can also be used to drive a rotary drill bit or air hammer. However, in order to transport the drill cuttings out of the wellbore, the hydrostatic head of the fluid column can often exceed the pressure of the formation being drilled. Therefore, the drilling mud or fluid can invade into the formation, causing significant damage to the formation, which ultimately results in loss of production. In addition, the drill cuttings themselves can cause damage to the formation as a result of the continued contact with the formation and the drill cuttings. Air drilling with a rotary drill bit or air hammer can also damage the formation by exceeding the formation pressure and by forcing the drill cuttings into the formation.
Underbalanced drilling technology has been developed to reduce the risk of formation damage due to the hydrostatic head of the fluid column, which uses a mud or fluid system that is not weighted. Hence, drill cutting can be removed without having the fluid column hydrostatic head exceed the formation being drilled resulting in less damage to the formation. Underbalanced drilling techniques typically use a commingled stream of liquid and gas such as nitrogen or carbon dioxide as the drilling fluid.
Nevertheless, even when using underbalanced drilling technology, there still is the possibility of damage to the formation. The drilling fluid and drill cuttings are still being returned to the surface via the annulus between the drill pipe and the formation. Hence, some damage to the formation may still occur due to the continued contact of the drilling cuttings and fluid with the formation. As well, underbalanced drilling is very expensive for wells with low or moderate production rates.
Formation damage is becoming a serious problem for exploration and production of unconventional petroleum resources. For example, conventional natural gas resources are buoyancy driven deposits with much higher formation pressures. Unconventional natural gas formations such as gas in low permeability or “tight” reservoirs, coal bed methane, and shale gases are not buoyancy driven accumulations and thus have much lower pressures. Therefore, such formations would damage much easier when using conventional oil and gas drilling technology. There was a need for a drilling method that reduces the amount of formation damage that normally results when using air drilling, mud drilling, fluid drilling and underbalanced drilling.
Two such methods have recently been disclosed in U.S. Patent Applications Publication Nos. 20030173088 and 20030155156, incorporated herein by reference, using concentric drill pipe and concentric coiled tubing, respectively. The methods each comprise the steps of (a) providing a concentric drill string having an inner pipe or tube situated within an outer pipe or tube defining an annulus between the two pipes or tubes, (b) connecting a drilling means at the lower end of the concentric drill string, and (c) delivering drilling medium through one of the annulus or inner pipe or tube and removing the exhausted drilling medium and entrained drill cuttings by extracting the exhausted drilling medium through the other of the annulus or inner pipe or tube.
These methods for drilling a wellbore can further comprise the step of providing a downhole flow control means positioned near the drilling means for preventing any flow of hydrocarbons from the inner pipe or tube or the annulus or both to the surface when the need arises. When using concentric drill pipe, the flow control means will also operate to shut down the flow from both the inner pipe and the annulus when joints of concentric drill pipe are being added or removed.
A downhole flow control means can also be used when testing a well for flow of hydrocarbons and the like during the reverse circulation drilling process. During drilling, the downhole flow control means is in the complete open position to allow for the reverse circulation of the drilling fluid, i.e., drilling fluid can be pumped down either the annulus or inner space of the inner pipe or tube and exhausted drilling fluid and drill cuttings are removed through the other of said annulus or inner space. However, when testing is required during the reverse circulation drilling process, the wellbore annulus is sealed off and the downhole blowout preventor seals off either the annulus or the inner space. Thus, the material to be tested can flow to the surface through the other of the annulus or inner space.
There is a need for a downhole flow control means or a downhole blowout preventor for use with concentric drill string that is fast, easy and safe to use.